| Incised valleys resulting from fluvial erosion in response to sea-level fluctuations are distributed widely around the world throughout geological history.Their fills are critical stratigraphic archives for unraveling long-term earth-surface processes,depositional history,sea-level change,sedimentation,and the interaction between terrestrial and coastal marine processes in ramp and shelf settings,as well as for documenting environmental and climatic changes.In addition,the investigation of incised-valley fills is important for an improved understanding of sequence-stratigraphic divisions which provide a basis for global correlation of stratigraphic units of marine-to-continental successions.Previous studies predominantly concentrated on the wave-dominated or mixed-energy(wave and tidal)incised valleys,and few models have been established for tide-dominated systems.Also,tide-dominated estuaries have received less attention and are more poorly documented than wave-dominated ones because they are less common and more difficult to investigate.The present-day Qiantang River(QR)estuary is a world-famous tide-dominated estuary,and is one of the most sensitive areas for postglacial global sea-level changes in China.Its incised valley has been studied for more than three decades.However,although there has been much descriptive documentation of the architecture for the QR incised-valley fill and its paleoenvironmental regimes,a number of scientific problems remain.This paper is an extension of the previous work on the QR incised valley.Based on detailed observations of the newly acquired SE1 and SE2 boreholes,including lithology,sedimentary texture and stuctre,succession character,mineral composition,foraminifera,magnetic susceptibility,grain size,element geochemistry,and 14C dating,as weln as the stratigraphic correlation with more than 800 boreholes,we aim to 1)describe in detail the sedimentary facies with special emphasis on identifying and illustrating the stratigraphic surfaces and paleo-estuary facies,as well as reconstructing a more elaborate sedimentary succession for the late Quaternary QR incised-valley fill;2)characterize the sedimentary response to changes in sea level,sediment supply and sedimentary dynamics;3)develop a depositional model for the QR incised-valley and estuary fill,and compare the paleo-and present-day estuarine deposits;and 4)discuss the evolution of sediment supply throughout the QR incised-valley filling and the "coupling relationship" with the adjacent Changjiang River incised valley.Filling of the QR incised valley was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea-level stillstand.The early part of the fill is transgressive in character,while the younger part is regressive.From bottom up it could be grouped into eight sedimentary facies:natural levee,fluvial channel,floodplain,paleo-estuary,offshore shallow marine(and/or littoral facies),and present-day estuary.This succession is also a characteristic of the postglacial incised-valley fills in Asian regions,which were deposited during the last postglacial sea-level rise and subsequent stillstand.This is the first time the paleo-estuary facies has been recognized.It has a distinct sedimentary succession from the present-day estuary,and is characterized by a large number of tidal-channel sand bodies flanked by mudflats and lacustrine marshes in the landward part of the estuary,passing headward into the tidal-fluvial channel.The present-day estuary is mainly composed of bay-mouth sediments,as well as a complex of tidal sand ridges and channels at the mouth,followed headward by a silty bar corresponding to the area of highest tidal energy,which passes landward to a single tidal-fluvial channel.The sediments of paleo-estuary present a coarse-fine-coarse seaward pattern,in which the finest sediments occur in the beadload convergence zone(BCZ).By comparison,the deposits of present-day estuary show a coarse-fine-coarse-fine pattern from land to the sea,with fine-grained sediments present in both the BCZ and bay-mouth areas,the later being finer than the former.This pattern is in general similar to the grain-size distribution in tide-dominated estuaries,but a significant difference exists in that the bay-mouth sediments of the present-day estuary are mud-dominated.According to the mineral composition and element geochemistry,the provenance for fluvial and paleo-estuarine sediments is completely different from that of offshore shallow marine and present-day estuary.The former sediments are mainly derived from the provenance characterized by meso-acid volcanic rocks,delivered by the paleo-QR.While the offshore shallow marine and present-day estuarine sediments predominantly come from the meso-basic provenance with some meso-acid volcanic rocks,which is different from the provenance type identified by CH2 borehole as well as that of Changjiang and Changjiang delta sediments.This may be related to the position of SE2 borehole located in the landward part of estuary where the influence of local provenance is more intense.Since the last glaciation there have been three stages for the development of QR incised-valley fill sequence:1)formation stage(20000-15000 yr B.P.)associated with sea-level fall,2)rapid filling stage(15000-7500 yr B.P.)during the early transgression,and 3)burial stage(7500 yr B.P.?present)associated with the slowing of sea-level rise and the onset of progradation.It consists of four longitudinal segments,each of which is characterized by a distinct stratigraphic succession,and different degrees of marine and fluvial influences.The seaward segment extends from the most seaward extent of valley incision to the headward limit of offshore shallow-marine facies at the present time.It is characterized by aggrading and backstepping fluvial lag deposits of the LST,fluvial-channel,fluvial channel-floodplain complex and paleo-estuary deposits of the TST,overlain by offshore shallow-marine deposits of the TST and early HST.The second reach lies between the inner end of segment 1 and the headward limit of offshore shallow-marine facies at the time of maximum transgression.Compared to the segment 1,the present-day estuary develops in the top part and its nature varys along the length of this segment.The third reach corresponds to the area occupied by the drowned-valley estuary at the end of transgression.In this segment,the offshore shallow-marine sediments are absent,and the estuarine sediments are capped by the highstand fluvial sediments with the nature varying along the length of this segment.The innermost reach is developed headward of the transgressive estuarine limit and extends to the point where relative sea-level change no longer controls fluvial style.This segment is characterized by fluvial deposits throughout its depositional history.The QR incised valley deposits occur as a relatively complete type I sequence.The basal erosion surface and walls of the incised valley,as well as the top of paleosol on the interfluves and the fluvial-terrace deposits comprise the sequence boundary(SB).The initial flooding surface lies within the lower part of fluvial succession in the axis of QR incised valley.On the interfluves and fluvial terraces,it coincides with the SB.The maximum flooding surface(MFS)is located within the offshore shallow-marine facies.At this interface,the abundance of species and individuals,and compound differentiation of benthonic foraminifera,as well as the TOC reach the maximum values.In the headward part,the MFS is put at the point where upward fining changes to upward coarsening.Tidal-ravinement surfaces are commonly present in the paleo-estuary facies,and tidal-erosion surfaces are present in the present-day estuary facies.The wave ravinement surfaces typifing wave-dominated or mixed-energy incised-valley fills are absent in the macrotidal QR succession,and tidal ravinement and erosion surfaces are more extensive and numerous.The filling of QR estuary and incised valley was mainly controlled by relative sea-level change,sedimentary processe,sediment supply and coastal topography.The relative sea-level changes determine the stratal stacking patterns,but local environmental factors,such as physical processes,sediment supply,and coastal configuration control the nature of facies and sediment distribution patterns.For macro-tidal nature,the morphosedimentary organization of the QR estuaries differs significantly from the typical tripartite sandy barrier-inlet complex/muddy central basin/bayhead delta distribution that typifies wave-dominated estuaries.The fluvial sediments of QR incised valley were mainly brought by the paleo-QR and its tributaries from their hinterland.The presence of foraminifera and tide-influenced sedimentary structures within the paleo-estuarine deposits suggests that the QR incised valley began to receive the sediments from the estuary mouth by flood-tidal currents,and the amount increases towards the upper part.The marine-sourced sediments may come from the continental shelf that was overlain by fine sand sediments during the paleo-estuary period,and then by the fine-grained sediment(silt,clayey silt and clay)in the present-day estuary period supplied by Changjiang River.This is likely the reason why tidal-channel sandy bodies are less abundant in the present-day estuary,even though the two estuarine phases has similar depositional condition.Some tidal sand ridges are present in the present-day estuary,but they are composed of coarse-grained sediments derived by tidal erosion from the underlying incised-valley fill.The sediments of tidal-fluvial channel and tidal point bars are mainly river-supplied.Coastal configuration,through its control on tidal dynamics,is a critical factor determining the character of incised-valley deposits.The QR incised valley is associated with the tide-dominated East China Sea,and the elongated and funel-shaped valley configuration is favorable for the development of hypersynchronous tidal behavior.Consequently,the paleo-and present-day estuaries are most likely tide-dominated.Additionally,the incision depth governs the potential for preservation of the infill.The QR incised valley with a maximum depth of 125 m at the mouth contains a relatively complete succession. |
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